CompilerMSL add emit_custom_functions() function.
CompilerMSL restrict use of as_type<> cast to necessary conditions.
CompilerMSL refactor get_declared_struct_member_size() and
get_declared_struct_member_alignment() functions, and remove
unnecessary get_declared_type_size() functions.
Add test shaders-msl/vulkan/frag/spec-constant.vk.frag.
Add to suite of MSL tests and references any existing GLSL tests
that successfully convert GLSL->SPIRV->MSL and compile as MSL.
test_shaders_helper() ignores hidden files that start with '.',
to avoid accidentally finding hidden OSX files such as .DS_Store.
Use xcrun to compile MSL shaders instead of hard-coded path to Metal compiler.
Wrap calls to xcrun in exception handling to ignore if Xcode not installed.
For MSL tests, move call to validate_shader_msl() to after call to
regression_check() to allow a converted MSL shader to be saved for
manual review even if it doesn't successfully compile as MSL.
